The present invention relates to the field of power-line carrier communications, more specifically to an apparatus and method for communicating among transceivers separated by power distribution transformers.
Power-line carrier (PLC) systems provide a means for electronically communicating between two points using the existing wiring of a power distribution system. PLC systems are common for communicating within a building, whether the building is a house, apartment building, business, or industrial building. Each point on the power lines within the building is directly connected to each other point in the building, except in the rare case where a distribution or isolation transformer is installed inside a building. More typically, however, power is supplied from a distribution transformer installed on a power pole to the building.
In distributing power from a utility generating station to a utility subscriber's building, the power passes through successively smaller branching networks, until it is finally split off to several distribution transformers, each of which have secondary windings directly connected to one or more subscribers. Because each of the subscribers connected to a single distribution transformer have power lines electrically connected to the secondary side of the same distribution transformer, PLC communication is easily possible between subscriber sites which share a common distribution transformer, but communication beyond the typically small number of subscribers on one distribution transformer requires passing a PLC signal through or around one or more distribution transformers.
One application of a PLC system is for remote meter reading where meters at each subscriber site record usage and transmit a signal indicating the amounts used to a local receiver, which collects reading from several transmitters using PLC communications, and then relays the data to a central utility computer via radio or telephone lines. The transmitters can also be configured as receivers to receive data such as load shedding and power blocking commands from a local transceiver. With one local transceiver or receiver connected at the secondary side of each distribution transformer, no PLC communications through a distribution transformer is necessary. However, such a system is uneconomical, and in a practical system, a local transceiver must be able to collect data from and distribute data to subscribers on more than one distribution transformer, thus requiring an ability to communicate through one or more distribution transformers.
Since the distribution transformers are optimized to pass power at the power line frequency, typically 60 Hz, a PLC carrier frequency at a higher frequency is greatly attenuated by the transformer. A higher frequency PLC carrier is necessary to achieve adequate data rates and to allow the PLC signal to be separated from the power being delivered to the subscriber's power lines. Loss of signal through distribution transformers is especially troublesome when communicating between two subscribers coupled to separate distribution transformers, since signals must pass through both distribution transformers and therefore the loss is squared.
One method of transmitting higher frequency PLC carriers through a distribution transformer is to bypass the transformer. For example, U.S. Pat. No. 4,473,817, issued to Perkins, illustrates a communication system where a signal is passed from the primary side of a distribution transformer to the secondary side using a separate communications transformer with a capacitor in a circuit optimized for the carrier frequency. However, such a system requires additional hardware and the second transformer absorbs some of the power destined for subscribers.
From the above, it is seen that an improved means for communicating through a distribution transformer is needed.